A conventional microwave/millimeter wave sensor apparatus is formed by connecting individual functional circuits, such as a transistor oscillating circuit or a GUNN diode oscillating circuit, a diode mixer circuit, an antenna, a coupler, a divider, and a transmission-reception separating circulator, with the use of a transmission line such as a microstrip line. Such a conventional microwave/millimeter wave sensor apparatus normally uses an oscillation signal of the oscillating circuit as a transmit RF signal, and withdraws part of the oscillation signal at the divider circuit and uses the withdrawn signal as a mixer circuit local signal. With the mixer functioning as a homodyne downconverter, this microwave/millimeter wave sensor apparatus is a homodyne sensor type apparatus that converts a receive RF signal to an IF signal.
In such a microwave/millimeter wave sensor apparatus formed by connecting individual functional circuits, there is a limit to high-density integration of circuits. Particularly, in millimeter wave bands, the loss due to the connecting portions between the functional circuits and the loss due to the transmission line become main causes of circuit performance degradation. To counter this problem, homodyne sensors and homodyne downconverters each having an oscillating circuit, a mixer circuit, an antenna, and the like integrated into one structure have been suggested.
For example, Non-patent Literature 1, “C. M. Montiel, ‘A self-mixing active antenna for communication and vehicle identification applications’, MTT-S Digest, 1996”, discloses a structure in which a GUNN diode to be used as an oscillating device and a mixing device is mounted directly inside a round conductor patch, and an IF signal is withdrawn from a bias Tee circuit equipped with a DC block capacitor connected to the round conductor patch.
Non-patent Literature 2, “Robert A. Flynt, ‘Low Cost and Compact Active Integrated Antenna Transceiver for System Application’, MTT-10 vol. 44, October, 1996”, discloses a structure in which a FET to be used as an oscillating device is placed at the center of two half-round conductor patches capacitatively coupled to each other by a chip capacitor, and a Schottky barrier diode to be used as a mixing device is mounted directly inside the drain-side conductor patch.
Non-Patent Literature 3, “M. J. Kelly, ‘HBT active antenna as a self-oscillating Doppler sensor’, IEE Proc., Microw. Antennas Properg., vol. 147, No. 1, February, 2000”, discloses a structure in which a regular microstrip line transistor oscillating circuit for 50-ohm load designed independently of a conventional rectangular conductor patch antenna is connected on the same plane to the feeding point of the rectangular conductor patch antenna of 50 ohms in feed impedance, the transistor is used as an oscillating device and a mixing device, a receive RF signal is input to a portion between the collector and the emitter from the conventional rectangular conductor patch antenna, a 20-ohm resistor is placed between the drain-side RF choke of the transistor and a direct current source, and an IF signal is withdrawn as a voltage from a terminal on the choke side of the resistor.
In the structure having a GUNN diode mounted directly inside the round conductor patch as disclosed in Non-patent Literature 1, however, the DC-RF conversion efficiency of the GUNN diode is much lower than that of a transistor, though there is no power loss due to the transmission line. As a result, the power consumption becomes larger, and a stable operation cannot be expected unless a structure having a high heat radiation rate is used. Also, the GUNN diode cannot achieve higher RF-IF conversion gain than a transistor. Therefore, the GUNN diode is disadvantageous in terms of detection sensitivity. Although a planar conductor patch radiation structure is used in the invention disclosed in Non-patent Literature 1, this conventional art cannot provide a sensor apparatus that has a simple structure, is less expensive, and has high power efficiency.
In a structure in which a FET to be used as an oscillating device is placed at the center of two half-round conductor patches capacitatively coupled to each other by a chip capacitor, and a Schottky barrier diode to be used as a mixing device is mounted directly inside the drain-side conductor patch as disclosed in Non-patent Literature 1, a plurality of RF-band components are placed in the conductor patches, resulting in a complicated structure. Moreover, capacitative coupling by a chip capacitor can be realized hardly in millimeter wave bands. Since a Schottky barrier diode is used for mixing, higher RF-IF conversion gain than that of a transistor cannot be expected, and this structure is disadvantageous in terms of detection sensitivity.
In a structure in which a microstrip line transistor oscillating circuit designed for 50Ω load is merely connected on the same plane to the feeding point of a rectangular conductor patch antenna designed for 50Ω feeding as disclosed in Non-patent Literature 3, inevitable coupling is caused between the rectangular patch antenna and the microstrip line conductor of the oscillating circuit. As a result, the conductor pattern of the oscillating circuit affects the radiation output, the radiation pattern, and the oscillation frequency characteristics. Having such a drawback, the invention disclosed in Non-patent Literature 3 is not easily put into practice. Moreover, the invention disclosed in Non-patent Literature 3 is a structure in which a regular microstrip line rectangular patch antenna of 50 ohms in input impedance is connected to a regular microstrip line oscillating circuit of 50 ohms in output impedance, and the antenna and the oscillating circuit do not exist in an integrated manner. Particularly, in millimeter wave bands, the power loss due to the microstrip line portion forming the feedback circuit or the like for oscillation becomes larger, and therefore, this conventional art is disadvantageous in terms of efficiency.
In view of the above circumstances, the present invention aims to provide a microwave/millimeter wave sensor apparatus that can acquire highly-sensitive sensing information (IF signals for performing operation detection, velocity detection, existence detection, location detection, and the like), while being a simple structure, being inexpensive, and having high power efficiency.